This invention relates to turbines, which term is used in this specification to mean rotary machinery which is driven by or which drives a working fluid such as gas or steam by means of rings of blades mounted on a rotor to intercept the working fluid in an annular passage. The invention is of particular application to gas turbine compressors for providing compressed air to the fuel combustion process, but it is not limited to this application.
A gas turbine compressor has a rotor with rings of blades spaced along the axis so as to interleave with rings of fixed blades extending inwardly from the compressor casing. This invention is concerned with the rotor blades and the smooth flow of fluid (air) from each ring of blades to the next.
On a compressor rotor the moving blades are often fitted with dovetail roots which engage complementary grooves in the respective rotor disc so holding the blades against the very large centrifugal forces that a rise. The dovetail section blade root may be fitted to the rotor in circumferential grooves, or more commonly, grooves which are axial or skewed to the rotor axis according to the particular aerofoil section of the blade.
The machining of the dovetail grooves in the rotor, where these grooves are axial or skewed to the axis, requires circumferential recesses in the rotor in front of each ring of blades and to a radial depth below that of the roots of the downstream blades. The term front of is taken to mean "upstream of" and correspondingly "behind" is taken to mean "downstream of". In a compressor, as the flow moves downstream and the gas is compressed, the annular passage is made smaller by increasing the rotor diameter and reducing the height of the blades.
In a conventional machine this stepping of the rotor diameter assists the machining in that the inter-stage recess has to be only marginally deeper than the downstream blade root.
In any event the inter-stage recess would take the gas flow out of the direct path between the aerofoil Sections of the blades, the resulting turbulence causing losses. This is normally prevented by providing bridge pieces which effectively close off the recess and provide a smooth inner boundary to the flow, limiting it to the annular region of the blade aerofoil section.
One method of fitting such bridge pieces is to fit platform-like pieces in grooves in the upstream and downstream razes of the rotor discs close to the rim of the discs. The bridge pieces are fitted into the grooves through a machined window and then slid around the grooves to their required positions.
Allowance is made for expansion of the bridge pieces both axially and circumferentially and consequently seals are required. Additionally stop plates and springs may be required to be fitted to prevent uncontrolled circumferential sliding of the bridge pieces.
A further disadvantage of this method of providing bridge pieces is that the grooves on the faces of the rotor discs for receiving the bridge pieces necessitate an increase in the axial spacing of the stages beyond that required for entry of the blade root on assembly.
Again, the centrifugal force on the bridge pieces is carried as an additional load on the disc rim which is already highly stressed by the forces in the rotor blades.
An object of the present invention is to provide a turbine having bridge pieces which very largely avoid the above disadvantages.